Apparatus and method for measuring signal quality of a wireless communications link

ABSTRACT

An apparatus comprises a link interface to receive an OFDM symbol from a communications link. A signal-to-noise ratio estimation unit generates an estimate of a geometric signal-to-noise ratio for the received symbol based on a function of the soft and hard decisions. A signal quality of the wireless communications link is based on the geometric signal-to-noise ratio.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.10/322,130 filed on Dec. 17, 2002. The disclosure of the aboveapplication is incorporated herein by reference in its entirety.

TECHNICAL FIELD

The present invention generally relates to wireless communications, andmore specifically, to a method and apparatus for measuring signalquality of a wireless communications link which features geometricsignal-to-noise ratio estimation.

BACKGROUND

The past few years has witnessed the ever-increasing availability ofrelatively cheap, low power wireless data communication services,networks and devices, promising near wire speed transmission andreliability. One technology in particular, described in the IEEEStandard 802.11a (1999) and Draft IEEE Standard 802.11g (2002) High RatePHY Supplements to the ANSI/IEEE Standard 802.11, 1999 edition,collectively incorporated herein fully by reference, has recently beencommercialized with the promise of 54 Mbps effective bandwidth, makingit a strong competitor to traditional wired Ethernet and the moreubiquitous “802.11b” or “WiFi” 11 Mbps mobile wireless transmissionstandard.

IEEE 802.11a and 802.11g or “802.11a/g” compliant transmission systemsachieve their high data transmission rates through using OrthogonalFrequency Division Modulation or OFDM encoded symbols mapped up to 64QAM multicarrier constellations and beyond. Generally, OFDM worksgenerally by dividing one high-speed data carrier into multiple lowspeed sub-carriers which are used for transmission of data in parallel.Put another way, the data stream of interest is divided into multipleparallel bit streams, each transmitted over a different sub-carrierhaving a lower effective bit rate. Before final power amplification andtransmission, the multicarrier OFDM symbol encoded symbols are convertedinto the time domain using Inverse Fast Fourier Transform techniquesresulting in a relatively high-speed time domain signal with a largepeak-to-average ratio (PAR). OFDM is also used in fixed broadbandwireless access systems such as proposed in IEEE Standard 802.16a: AirInterface for Fixed Broadband Wireless Access Systems Part A: Systemsbetween 2 and 1 GHz, Draft working document, February 2002, (“802.16a”)which is incorporated herein fully by reference.

In the case of 802.11a and 802.11g, there are up to 52 definedsub-carriers or tones, of which 48 are available to carry data (the 4remaining are pilot sub-carriers or tones, which bear predetermineddata). These sub-carriers are substantially orthogonal to each other, sothey can be spaced closer together than in conventional frequencydivision multiplexing. Mathematically, the integral of the product ofany two orthogonal sub-carriers is zero. This property allows theseparating of sub-carriers at the receiver without interference fromother sub-carriers.

Even where wireless communications leverage orthogonal sub-carriertechniques such as provided in 802.11a/g, they are still subject toenvironmental effects which can distort, disrupt or inject noise,thereby at least intermittently impact effective data throughput orother aspects of communications link quality or performance. Thus,planning and accommodating for foreseeable environmental effects is animportant part of wireless communications system design, and providing asystem that is responsive to such effects and self-heals or adapts linkcharacteristics or operational parameters is desirable. In an 802.11a/gimplementation, link characteristics such as data transmission rate,convolutional coding rate, packet/frame size, transmit power, receivergain, and frequency selection can be altered to preserve the link indeteriorating conditions, as well as to upgrade effective datathroughput in the link when conditions improve. To this end, it would beadvantageous to provide a wireless communications system capable ofassessing communications link quality or performance, and adapt linkcharacteristics in accordance with such assessed quality or performance.Further, It would be advantageous if a measure of such link quality orperformance can be accurately obtained in a computationally efficientmanner, to provide a parameter useful for efficient link management andadaptation.

SUMMARY OF THE INVENTION

The present invention is directed in part to an apparatus and method formeasuring signal quality in a communications link that supports transferof OFDM symbols modulating data across a plurality of sub-carriers. Theapparatus includes a link interface capable of receiving an OFDM symbolfrom the communications link, and a signal-to-noise ratio (SNR)estimation unit to generate an estimate of a geometric SNR for the OFDMsymbol based on a function of a soft decision and a hard decision forthe OFDM symbol. A signal quality of the communications link is based onthe geometric SNR estimate. This arrangement results in acomputationally efficient yet accurate way to assess link signalquality, and selectively perform link characteristic alteration based onthe SNR estimate.

In accordance with at least one disclosed embodiment of the invention,only a subset of the plurality of sub-carriers need be used to derive arelatively accurate geometric SNR estimate for signal quality assessmentpurposes. The subset need not be ordered or spaced apart in thesub-carrier constellation in a particular manner, as long as asufficient number of sub-carriers are selected as subset members toconstitute a representative sample of all the sub-carriers. This resultsin an even more computationally efficient technique to assess signalquality.

Moreover, in accordance with at least one disclosed embodiment, softdecisions of the likely transmitted symbols used to calculate theestimated geometric SNR may be provided by a demodulation/frequencydomain equalizer (FEQ). Though not required, this demodulation unit/FEQmay be arranged such that several of the calculation units may be sharedbetween it and the SNR estimation unit, as several of the involvedcalculations are similar.

Further, in accordance with at least one disclosed embodiment, harddecisions of the likely transmitted symbols used to calculate theestimated geometric SNR may be provided by either a slicer or a Viterbidecoder followed by an OFDM re-coder. The slicer alternative providesthe hard-decision relatively quickly, whereas the Viterbi decoderalternative is at least potentially more accurate in predicting thetransmitted symbol.

These and other aspects of the invention may be convenientlyincorporated into a baseband processor, transceiver, network interfaceapparatus such as or an information processing apparatus such as acomputer. Additional aspects and advantages of this invention will beapparent from the following detailed description of embodiments thereof,which proceeds with reference to the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a functional block diagram of a transceiver according to anembodiment of the invention.

FIG. 2 is a more detailed functional block diagram of the receiverbaseband processor shown in FIG. 1.

FIG. 3 is a more detailed functional block diagram of thesignal-to-noise ratio estimation unit shown in FIG. 2.

FIG. 4 is flowchart illustrating geometric SNR estimation and linkadaptation according to an alternative embodiment of the invention.

DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION

FIG. 1 illustrates a wireless communications transceiver 100 accordingto an embodiment of the present invention, including the receiverbaseband processing unit 120 shown in more detail in FIG. 2. In thisembodiment, inbound RF signals conveying a 802.11a/g compliant PLCPframe of OFDM encoded symbols are picked up by the duplex antenna 110and routed to the RF receiver unit 115 of a receiver 150 consistent withthe present invention. This frame originated from another 802.11a/gtransceiver, such as remote station 180. If the remote station 180originated transmission of the frame, RF signals conveying the framewould radiate from antenna 181 of the station, cross a wireless medium(not shown) interposing the station 180 and the transceiver 100 andwould be picked up by the antenna 110.

The RF receiver unit 115 performs routine downconversion and automaticgain control of these inbound RF signals, and presents an analogbaseband signal containing at least one frame of 802.11a/g OFDM symbolsto the receive baseband processor 120. Generally speaking, the receivebaseband processor 120 performs symbol demodulation of the each inbound802.11a/g compliant frame to recover bitstream data for receiversynchronization (preamble), frame or packet definition (header), or theactual inbound data of interest (payload). As will be described in moredetail below with reference to FIGS. 2 and 3, the receive basebandprocessor 120 includes a geometric signal-to-noise ratio (SNR_(geo))estimation unit 235 to measure signal quality and provide a signalquality parameter (SQ) to the MAC layer controller 128 of the MACinterface 125, a type of network interface bridging the OSI layer 1 PHYtransceiver with higher layer (OSI 2+) networks and applicationsserviced by the transceiver. The SQ parameter is indicative of theperceived or measured signal quality of the 802.11a/g wirelesscommunications link established between the transceiver 100 and theoriginator of the RF signals conveying the inbound OFDM frame, such asan 802.11a/g access point or another 802.11a/g client transceiver.Though not required, in this embodiment, SQ is equal to the SNR_(geo)estimate on the decibel scale. In other embodiments with different SQassessment considerations, SQ may correspond to or otherwise be basedon, but not necessarily be equal to the SNR_(geo) estimate.

Once recovered by the receive baseband processor 120, the inbound datacontained in each received 802.11a/g formatted frame (i.e. the framepayload or PSDU) is delivered to a the MAC layer interface 125 in theform of a MAC layer PPDU (a type of data unit) and then on to higherlayer applications and devices being serviced by the transceiver 100.Outbound data intended for wireless transmission originating from thedevice(s) or application(s) being serviced by the transceiver 100 aredelivered to the transmit baseband processor 135 of the transmitter 160from the MAC interface 125 using, e.g. one or more MAC layer PPDUs. Thetransmit baseband processor 135 formulates appropriate 802.11a/g framepreamble and header information, and OFDM symbol encodes the outbounddata to generate one or more complete outbound 802.11a/g frames. As theframe or packet is being developed, it is converted into analog formsuitable for upconversion and RF transmission by the RF transmitter unit140 consistent with 802.11a/g physical layer requirements.

The MAC layer controller 128 handles PLCP functions consistent with thepresent invention as well as specified in the 802.11a/g standards,including at least periodically monitoring quality of the wirelesscommunications link between the PMD portion of the transceiver (i.e. thereceiver 150, antenna 110, and transmitter 160) and the remotestation(s) the transceiver 100 either sends PLCP frames to or receivesPLCP frames from, such as the station 180 shown in FIG. 1. This wirelesscommunications link, conceptually shown in FIG. 1 (reference numeral185) spanning station 180 and the transceiver antenna 110, defines atleast one static or dynamically allocated carrier RF frequency orchannel within a set of available channels defined in the 802.11a and/or802.11g standards which may be used to bear the PLCP frame trafficbetween the station 180 and the transceiver 100. The communications link185 also defines a number of characteristics relating to how the PLCPframes are to be transmitted, including the agreed upon convolutionalcoding and data transmission rates, packet length (particularly the sizeof the PPDU or payload conveyed per frame), as well as transmitter powerused to radiate the outbound frames. The communications link furtherincludes individual receiver characteristics, including receiver gain orsensitivity settings and receive antenna selection (in a multiplereceiver antenna deployment—not shown in FIG. 1).

Consistent with 802.11a and 802.11g standards, the MAC layer controller128 can selectively alter link characteristics of the communicationslink 185 on an ongoing basis based on, e.g. perceived signal strength,channel availability and/or interference on an ongoing basis through theuse of known 802.11a/g primitives. In so doing, the MAC layer controller128 may issue 802.11a/g primitives to the transceiver PMD causing thePMD to locally alter link characteristics as well form and cause the PMDto issue frames over the communications link 185 to the remote station180 to induce changes in link characteristics at the remote end.Moreover, in the present embodiment, the MAC layer controller 128 alsoincludes logic to evaluate the SQ parameter supplied by the SNR_(geo)estimation unit 235 (FIGS. 2 and 3) and selectively alter one or morecommunications link characteristics in response thereto. For example,the MAC layer 128 may track the change in a scalar SQ value over timeand cause the transceiver PMD and/or the remote station to increase thedata transmission rate and, consequently, the effective data throughputof the communications link, if the SQ parameter increases in value (e.g.ratchet up effective throughput from 24 Mbps to 54 Mbps throughselecting use of a 64-QAM modulation constellation instead of a 16-QAMmodulation constellation), as well as back off such throughput when SQfalls over time. This is because, according to the present embodiment, arelatively high SQ parameter indicates a very clean (or relativelynoiseless) communications link that can tolerate maximum effective datathroughput without appreciable data loss, as the SQ parameter here isdirectly related to an estimate of the SNR_(geo) for the communicationslink, such as the link 185, measured at the receiver. Likewise, a low SQparameter indicates a low SNR_(geo) and the presence of a noisyreception environment and/or reduced inbound signal strength at thereceiver. In such case, a less aggressive transmission rate may provideacceptable decoding accuracy and processing gain.

Referring again to FIG. 1, it should be appreciated that only a singleduplex antenna arrangement is shown in FIG. 1, the transceiver 100 canbe easily adapted to incorporate multiple receive pathways or chains totake advantage of selection diversity or MRC diversity techniques.Likewise, though not shown in FIG. 1, transmit diversity techniques maybe employed in addition or in the alternative as would be understood bythose skilled in the art.

Also, though not shown in FIG. 1, the transceiver 100 may form anoperational part of a network interface apparatus such as a PC card ornetwork interface card capable of interfacing with the CPU orinformation processor of an information processing apparatus such as adesktop or laptop computer, and may be integrated within and constitutea part of such information processing apparatus. This network interfaceapparatus may alternatively form an operational component of a wirelesscommunications access point such as a base station as will beappreciated by those ordinarily skilled in the art.

As noted above and consistent with IEEE 802.11a/g standards, thereceiver 150 of the transceiver 100 of the present embodiment includesan RF receiver 115 to handle RF downconversion and filtering of receivedsignals in one or more stages and a receive baseband processor 120 todemodulate OFDM encoded symbols bearing the data of interest present inthe analog baseband signal recovered by the RF receiver 115. Thisreceive baseband processor 120 now be explored in more detail withreference to FIG. 2. Here, the recovered analog baseband signal z(t)generated by the RF receiver 115 is provided to the input of the I/Qdetector 202 to recover analog in-phase (i) and quadrature-phase (q)signals, which are then fed to the analog to digital converter 210.These i and q signals are then converted into their respective digitalcounterpart signal components I and Q, with each bearing digital data inthe time domain. Next, the I and Q components are sent to the FFT 208for conversion into the frequency domain and recovery of the OFDMsymbols present therein. In fact, the FFT 208 recovers the 52 modulatedsub-carrier signals Y₁ . . . Y₅₂ or forming each received OFDM symbolborne by the time domain I and Q signals. As used herein, the notationY_(n) represents the nth OFDM symbol within the received PLCP frame, andY_(n,k) represents the kth individual sub-carrier signal forming suchOFDM symbol Y_(n). The relationship between each sub-carrier of a givenOFDM signal Y_(n) and it's transmitted counterpart symbol sub-carrier(X_(n,k)) may be generally expressed as:Y _(n,k) =H _(n,k) X _(n,k) +N _(n,k)  (1)

where H_(n,k) is the actual channel response or channel estimate forsub-carrier Y_(n,k,) and N_(n,k) represents intervening noise, includingAdditive White Gaussian Noise present in the kth sub-carrier or toneduring transmission of the nth OFDM symbol.

After each OFDM symbol is recovered by the FFT 208, soft decisiondemodulation (phase rotation) and Frequency domain EQualization (FEQ)through a demodulation/FEQ unit 220 is performed on the FFT output tocompute a soft decision approximation of its originally transmittedcounterpart, denoted as X_(n)′ in the case of received symbol Y_(n). Inparticular, the FEQ 220 computes the following:

$\begin{matrix}{X_{n,k}^{\prime} = {\frac{Y_{n,k}}{H_{n,k}^{\prime}} = \frac{H_{n,k}^{\prime*}{\cdot Y_{n,k}}}{{H_{n,k}^{\prime}}^{2}}}} & (2)\end{matrix}$

for each of the sub-carriers (k=1 . . . 52) forming the symbol Yn. In astraightforward implementation, this computation requires ˜2 complexmultiplications and one real division per sub-carrier. H_(n,k)′ is theestimated channel response or channel estimate for sub-carrier Y_(n,k,)and N_(n,k) represents intervening noise, including Additive WhiteGaussian Noise present in the kth sub-carrier or tone duringtransmission of the nth OFDM symbol. Note that this is computationallymore intensive than implementing the FFT 208 that requires 96 complexmultiplications per 52 carriers. The implementation of a divisionoperation can be at least 3˜5 times more complex than that of amultiplication operation. Accordingly, as will be discussed in furtherdetail with reference to FIG. 3, the SNR_(geo) estimation unit 235 mayshare the 52 sub-carrier divider block (e.g. block 312) with the FEQ220.

Once the soft decision X′_(n) is computed, it is routed to a Viterbidecoder which uses known Viterbi algorithm techniques, includinghistorical analysis of channel traffic, to find the most likelytransmitted bitstream (b₀ . . . b_(t)) corresponding to the transmittedOFDM symbol Xn, which, depending on intervening noise such as N_(n), mayactually be either be the actual bitstream used to encode transmittedsymbol X_(n) or a close approximation thereof, as is known in the art.Though not shown in FIG. 2, the most likely transmitted bitstreamproduced by the Viterbi decoder is then descrambled using known802.11a/g techniques to recover the data of interest which is thenpassed to the MAC layer interface 125 in the form of a PPDU.

In this embodiment, though not required, hard OFDM symbol by symboldecisions dec {X_(n)′} or D_(n) of the soft decisions generated by theFEQ 220 are provided to the channel estimator 210 to improve the channelestimates H_(n)′ for successive symbols using known digital feedbackanalysis techniques. Moreover, these hard decisions are delivered to theSNR estimation unit 235 in order to compute an estimate of the SNR_(geo)which, consistent with the present invention, is used to measure thesignal quality of a wireless communications link in a computationallyefficient manner.

Generally speaking, the geometric SNR of an OFDM system may be expressedas:

$\begin{matrix}{{SNR}_{geo} = \left\lbrack {\prod\limits_{k = 1}^{N}\;{SNR}_{k}} \right\rbrack^{1/N}} & (3)\end{matrix}$

In other words, the geometric SNR for all sub-carriers in an OFDM systemis the geometric mean of the signal-to-noise ratio on each individualsub-carrier. As discussed above, since there are 52 sub-carriers used in802.11a/g communications, N=52 in this embodiment. From equation (1),the SNR for sub-carrier k of the nth OFDM symbol within a received802.11a/g PLCP frame may be expressed as:

$\begin{matrix}{{SNR}_{n,k} = {{\frac{H_{n,k}}{N_{n,k}}}^{2} = {{\frac{Y_{n,k}}{H_{n,k}} - X_{n,k}}}^{- 2}}} & (4)\end{matrix}$

However, neither the actual channel response H_(n,k), nor thetransmitted symbol X_(k) is known precisely, and in fact only estimatesof each (H′_(n,k), X′_(n,k) D_(n,k) or {circumflex over (X)}_(n,k)) areavailable. Therefore, SNR_(n,k) becomes:

$\begin{matrix}{{{SNR}_{n,k} \cong {\frac{H_{n,k}^{\prime}}{N_{n,k}}}^{2}} = {{\frac{Y_{n,k}}{H_{n,k}^{\prime}} - {K_{mod}\; D_{n,k}}}}^{- 2}} & (5)\end{matrix}$

and SNR_(geo) for the nth OFDM symbol in the received PLCP frame can beapproximated as:

$\begin{matrix}{{{SNR}_{geo} \cong \left\lbrack {\prod\limits_{k = 1}^{N}\;{{\frac{Y_{n,k}}{H_{n,k}^{\prime}} - {K_{mod}\; D_{n,k}}}}^{- 2}} \right\rbrack^{1/N}},{or}} & (6) \\{{SNR}_{geo} \cong {\left\lbrack {\prod\limits_{k = 1}^{N}\;{{X_{n,k}^{\prime} - {K_{mod}\; D_{n,k}}}}^{- 2}} \right\rbrack^{1/N}.}} & (7)\end{matrix}$

Here, Y_(n,k) represents the FFT output, H′_(n,k) represents the channelestimate generated by the channel estimator 210, X′_(n,k) represents thesoft decision generated by the FEQ 220, and D_(n,k) represents the harddecision generated by the slicer 225, for the kth sub-carrier of the nthOFDM symbol of the received PLCP frame. K_(mod) here is a normalizationconstant or scale factor applied to the hard decision based on thesub-carrier constellation being used, such as BPSK, QPSK, 16-QAM or64-QAM such that each of the sub-carriers have equal power.

Calculating the geometric mean of the difference between the soft andscaled hard decisions for each sub-carrier, as proposed in equations (6)or (7), offers a precise way of obtaining SNR_(geo). However, as wouldbe appreciated by those skilled in the art, implementing thiscalculation in high-speed hardware is relatively complex, and many ofthe calculation structures would be unique to an otherwise 802.11a/gcompliant baseband processor. Therefore, despite the benefits incurredthrough ascertaining SNR_(geo) on a per-symbol basis, implementingequations (6) or (7) would unacceptably raise product cost and powerconsumption, particularly in cost-sensitive mobile wirelesscommunications devices which constitute the present heart of the802.11a/g market. Therefore, consistent with the present invention, anapproximation of equations (6) or (7) is proposed which provides anacceptable approximation of SNR_(geo) on a decibel scale for signalquality estimation purposes, and ultimately a good measure of linkquality or link performance for the communications link in which theinbound PLCP frame has been transported. This approximation ismathematically expressed as follows:

$\begin{matrix}{{{SNR}_{{geo},n,{dB}} \approx {{Avg} \cdot \left\lbrack {{- 20}\;\log_{10}\;{{\frac{Y_{n,k}}{H_{n,k}^{\prime}} - {K_{mod}\; D_{n,k}}}}} \right\rbrack}},{k = {1\mspace{14mu}\ldots\mspace{14mu}{N.}}}} & (8)\end{matrix}$

In so doing, the geometric mean function expressed in equations (6) or(7) has been replaced by a more computationally efficient averagefunction and, at the same time, SNR_(geo) estimate is directly expressedin the familiar decibel scale.

Moreover, to further simplify the required computation overhead forSNR_(geo), it is further proposed that only a subset of the Nsub-carriers need be averaged in order to provide an acceptable estimateof SNR_(geo) for link quality assessment purposes. Though not required,it is suggested that the selected subset of sub-carriers be regularlyspaced within the symbol constellation, and that different subsets ofthe N sub-carriers be used in estimating SNR_(geo) in successive, thoughnot necessarily consecutive OFDM symbols within the same received PLCPframe. Alternatively, the same subset can be used for e.g. a given timeinterval, a given number of PLCP frames or symbols within a single frameif simplification of the sub-carrier selection process is desired.

For example, assume in an 802.11a/g OFDM system, a subset of Ksub-carriers are selected from a set of N sub-carriers. If K=8, meaningthat a subset of 8 sub-carriers are used to estimate SNR_(geo,n,) apossible regularly spaced subset could include sub-carriers {1, 8, 15,22, 29, 36, 43, 50} for the nth OFDM symbol in the received frame. Thus,in this case, every 7^(th) sub-carrier is selected for averaging. Forthe next OFDM symbol n+1, this subset could remain the same, oralternatively, a different subset, partially or fully distinct from theprevious subset, may be selected, such as {2, 9, 16, 23, 30, 37, 44,51}. Of course, this represents only a possible selection strategy toachieve an acceptable SNR_(geo) estimate, either in isolation or over anumber of received OFDM symbols or frames, and in fact other selectionstrategies may be implemented consistent with the present invention aslong as a sufficient number of sub-carriers are chosen to provide arepresentative subset of the symbol constellation.

With consideration given to such sub-carrier subset selection, equation(8) becomes:

$\begin{matrix}{{{SNR}_{{geo},n,{dB}} \approx {{Avg} \cdot \left\lbrack {{- 20}\;\log_{10}\;{{\frac{Y_{n,k}}{H_{n,k}^{\prime}} - {K_{mod}\; D_{n,k}}}}} \right\rbrack}},{k = \kappa_{0}},\kappa_{1},\ldots\mspace{14mu},\kappa_{K - 1},{K \leq {N.}}} & (9)\end{matrix}$

This relationship can be conveniently implemented by the SNR estimationunit 235 shown in FIG. 2 to provide an SNR_(geo) estimate, andconsequently a measure of signal quality SQ on a per received OFDMsymbol basis.

A more detailed view of the SNR estimation unit 235 in accordance withthe embodiment of FIG. 2 is shown in FIG. 3. Here, block 328 computesthe intermediate relationship U_(n) for the nth received OFDM symbol:U _(n,k) =K _(mod) ⁻¹ Y _(n,k) H′* _(n,k), k=1 . . . N  (10)

from the channel estimate H_(n)′ and the FFT output Y_(n). Note that, inso doing, the block 328 can be shared between the SNR estimation unit235 and the FEQ 220 (see e.g. equation (2) above=U_(n,k)/V_(n,k)). Block326 is likewise used to compute the square of the magnitude of Hn′, orintermediate relationship Vn, which may be expressed as:V _(n,k) =|H _(n,k)′|², k=1 . . . N  (11).

Like calculation block 328, calculation block 326 can be shared with theFEQ 220 used to compute X′n.

Still referring to FIG. 3, multiplication block 328 scales theintermediate relationship result U_(n,1) . . . U_(n,52) by K⁻¹ _(mod) toarrive at A_(n,1) . . . A_(n,52) or A_(n). Then subsets of both theV_(n) and A_(n) results are selected by the sub-carrier selection unit310. As described above, in this embodiment, the sub-carrier selectionunit selects a subset of N sub-carriers {κ₀, κ₁, . . . , κ_(K-1)}regularly spaced-apart in the symbol constellation. The number ofsub-carriers, K, selected can vary from 1 to N, but typically K rangesfrom 8 to 13 in 802.11a/g implementations to develop a reasonableestimate of SNR_(geo) while reducing the computation required inassessing each sub-carrier of the OFDM symbol. The A_(n,k) and V_(n,k)values corresponding to the selected subset are passed from unit 310 tothe division block 312 to compute A_(n,k)/V_(n,k), or X′_(n,k) (see e.g.equation (2)). Here, to cut down on overall circuit complexity, likecalculation blocks 326 and 328 discussed above, the division block 312may be conveniently shared with the soft decision unit or FEQ 220 usedto calculate X′_(n,k).

The QAM hard decisions D_(n,1) . . . D_(n,52) generated by the slicer225 are also fed to the sub-carrier selection unit 310. The Dn,k valuescorresponding to the selected subset {κ₀, κ₁, . . . , κ_(K-1)} arepassed to the scalar multiplication block 316, which multiplies each bythe constant Kmod. The resulting hard decision subset is then presentedto block 314 and subtracted from the results provided by the divisionblock 312, and the magnitudes are thereafter extracted. Then, theseresults are serialized (320), converted into the log domain by thelookup table 322 and averaged by the averaging block 324 to reach theSNR_(geo) estimate after K iterations. This SNR_(geo) is then sent tothe MAC layer controller 128 (FIG. 1) of the MAC interface 125 as asignal quality measure (SQ) for assessing the communications linkquality or link performance, and to selectively take correction actionresponsive thereto, using e.g. known MAC layer link characteristicalteration techniques such as decreasing or increasing data transmissionor convolution coding rates, as described above with reference to FIG.1.

It should be noted that, in this embodiment, intermediate results Un andVn are calculated for every subcarrier of the received OFDM signal sinceeach are needed for implementing the FEQ 220 which shares calculationblocks 326 and 328. Only those corresponding to the selected sub-carriersubset are passed (by unit 310) to the division block 312. In anotherembodiment, H′_(n) and Y_(n) may be filtered by unit 310 in advance ofthe U_(n) and V_(n) calculations to further reduce complexity at theexpense of some degree of sub-carrier selection flexibility as well aspotential calculation block sharing. Also, alternatively, calculationfor all sub-carriers may proceed to block 314, and then sub-carrierselection implemented by unit 310 may be applied immediately beforeserialization (block 320) in order to maximize circuitry, block, or unitsharing at the expense of redundant calculations.

Note here, that in the alternative to using the hard decision D_(n) inestimating SNR_(geo), one may alternatively use a re-coded post Viterbidecision {circumflex over (X)}_(n), in which the most likely transmittedbitstream is re-encoded into a encoded symbol {circumflex over (X)}_(n)using e.g. the re-coder 250 shown in FIG. 2 coupled to the SNRestimation unit 235. The use of re-encoded Viterbi results here doesimpart an additional delay in determining SNR_(geo) and SQ, but suchdelay can be accommodated without departing from the teachings of theinvention, especially since for link quality estimation purposes, theSNR_(geo) estimate does not change significantly per symbol. In fact, insuch case, substituting {circumflex over (X)}_(n) for D_(n) in equation(9):

$\begin{matrix}{{{SNR}_{{geo},n,{dB}} \approx {{Avg} \cdot \left\lbrack {{- 20}\;\log_{10}\;{{\frac{Y_{n,k}}{H_{n,k}^{\prime}} - {\hat{X}}_{n,k}}}} \right\rbrack}},{k = \kappa_{0}},\kappa_{1},\ldots\mspace{14mu},\kappa_{K - 1},{K \leq {N.}}} & (12)\end{matrix}$

While the above-described embodiments are implemented in hardwareincluding a combination of discrete and/or integrated logic, and/or oneor more application-specific integrated circuits, the teachings of thepresent invention are not meant to be so limited. In fact, as will beapparent to those skilled in the art, aspects and features of thepresent invention, including SNR_(geo) estimation as provided inequations (8) and (9) above, may be conveniently embodied in orimplemented by a programmed information processor, such as a generalpurpose microprocessor or microcontroller or a specific purpose device,such as a programmable digital signal processor. For example, FIG. 4depicts SQ estimation and selective link characteristic alterationprocessing according to an alternative embodiment of the invention. Inthis embodiment, though not required, the processing steps 410-440 arecarried out by an information processor programmed in accordance withsuch steps and in the general sequence shown, as would be recognized bythose skilled in the art. Note here that in step 415, calculation of SQmay be conveniently implemented through solving equation (8) highlightedabove. In yet another alternative embodiment not shown in FIG. 4,sub-carrier subset selection and SNR_(geo) estimation consistent withequation (9) is employed. In yet further embodiments, SQ calculation andSQ evaluation/characteristic alteration processing may be handled byseparate information processors or their functional equivalents.

It will be obvious to those having skill in the art that many changesmay be made to the details of the above-described embodiments withoutdeparting from the underlying principles of the invention. For example,though the above-described embodiments are directed to IEEE 802.11a/gwireless communications, the teaching of the present invention are notintended to be so limiting and can, in fact, be extended to otherwireless and wireline communications systems, such as communicationsystems compliant with OFDM xDSL broadband over copper and draft IEEE802.16a (2002) directed to broadband wireless access. The scope of thepresent invention should, therefore, be determined only by the followingclaims.

1. An apparatus, comprising: a link interface to receive an OFDM symbol from a communications link; and a signal-to-noise ratio estimation unit to generate an estimate of a geometric signal-to-noise ratio for the received symbol based on a function of soft and hard decisions, wherein a signal quality of the communications link is based on the geometric signal-to-noise ratio.
 2. The apparatus of claim 1 wherein the soft decision defines soft decision information for each of a plurality of sub-carriers, wherein the hard decision defines hard decision information for each of the plurality of sub-carriers.
 3. The apparatus of claim 2 wherein said signal-to-noise ratio estimation unit comprises: a sub-carrier selection unit to select a subset of the plurality of sub-carriers for the OFDM symbol.
 4. The apparatus of claim 3 wherein the subset of the plurality of sub-carriers are regularly spaced within a sub-carrier constellation defined by the plurality of sub-carriers.
 5. The apparatus of claim 4 wherein the sub-carrier selection unit selects first and second subsets of the plurality of sub-carriers for first and second symbols respectively, the first and second symbols successively transferred across the communications link, the first and second subsets being at least partially distinct.
 6. The apparatus of claim 3 wherein said signal-to-noise ratio estimation unit further comprises: an estimate calculator to generate the geometric signal-to-noise ratio estimate using soft and hard decision information for the subset of the plurality of sub-carriers selected by said sub-carrier selection unit as the soft and hard decision respectively.
 7. The apparatus of claim 1 wherein the communications link comprises a wireless communications link, and wherein the OFDM symbol is formatted in compliance with at least one of IEEE 802.11a and IEEE 802.11g standards.
 8. The apparatus of claim 1, wherein the hard decision is provided by a slicer communicatively coupled to said link interface and said signal-to-noise estimation unit, and wherein the soft decision is provided by a demodulation/FEQ unit communicatively coupled to said link interface and said signal-to-noise estimation unit.
 9. The apparatus of claim 1 wherein the hard decision is provided by a Viterbi decoder and symbol re-coder communicatively coupled to said link interface and said signal-to-noise estimation unit, and wherein the soft decision is provided by a demodulation/FEQ unit communicatively coupled to said link interface and said signal-to-noise estimation unit.
 10. A baseband processor comprising: an FFT unit to recover a received symbol from the baseband signal, the received symbol modulating data across a plurality of sub-carriers; a soft decision unit responsive to said FFT unit to recover a soft decision of a transmitted symbol corresponding to the received symbol; a hard decision unit responsive to said soft decision unit to recover a hard decision of the transmitted symbol based on the soft decision; and a signal-to-noise ratio estimation unit to generate an estimate of a geometric signal-to-noise ratio for the received symbol based on a function of the soft and hard decisions, wherein a signal quality of a communications link is based on the geometric signal-to-noise ratio.
 11. The baseband processor of claim 10 further comprising a channel estimator responsive to said FFT unit to provide a plurality of channel estimates for the received symbol corresponding to the plurality of sub-carriers, wherein said soft decision unit responds to said channel estimator to recover the soft decision using the plurality of channel estimates.
 12. The baseband processor of claim 10, wherein the soft decision defines soft decision information for each of the plurality of sub-carriers, wherein the hard decision defines hard decision information for each of the plurality of sub-carriers.
 13. The baseband processor of claim 12 wherein said signal-to-noise ratio estimation unit comprises: a sub-carrier selection unit to select a subset of a plurality of sub-carriers for the received symbol.
 14. The baseband processor of claim 13 wherein the subset of the plurality of sub-carriers are regularly spaced within a sub-carrier constellation defined by the plurality of sub-carriers.
 15. The baseband processor of claim 14 wherein said FFT unit successively recovers first and second received symbols from the baseband signal, each of the first and second received symbols modulating data across the plurality of sub-carriers, and wherein said sub-carrier selection unit selects first and second subsets of the plurality of sub-carriers, the first and second subsets being at least partially distinct.
 16. The baseband processor of claim 13 wherein said signal-to-noise ratio estimation unit comprises: an estimate calculator to generate the geometric signal-to-noise ratio estimate using soft and hard decision information for the subset of the plurality of sub-carriers selected by said sub-carrier selection unit, as the soft and hard decision respectively.
 17. The baseband processor of claim 16 wherein said estimate calculator and said soft decision unit share common calculation circuitry.
 18. The baseband processor of claim 10, wherein the communications link comprises a wireless communications link, and wherein the received symbol comprises an OFDM symbol compliant with at least one of IEEE 802.11a and IEEE 802.11g standards.
 19. The baseband processor of claim 10 wherein said soft decision unit comprises a demodulation/FEQ unit, and wherein said hard decision unit comprises one of a slicer and a combination of a Viterbi decoder and a symbol re-coder.
 20. A transceiver, comprising: a receiver capable of receiving a baseband signal borne across a wireless communications link; and a baseband processor responsive to said receiver and comprising: an FFT unit to recover a received symbol from the baseband signal, the received symbol modulating data across a plurality of sub-carriers; a soft decision unit responsive to said FFT unit to recover a soft decision of a transmitted symbol corresponding to the received symbol; a hard decision unit responsive to said soft decision unit to recover a hard decision of the transmitted symbol based on the soft decision; and a signal-to-noise ratio estimation unit to generate an estimate of a geometric signal-to-noise ratio for the received symbol based on a function of the soft and hard decisions, wherein a signal quality of the wireless communications link is based on the geometric signal-to-noise ratio.
 21. The transceiver of claim 20, further comprising a channel estimator responsive to said FFT unit to provide a plurality of channel estimates for the received symbol corresponding to the plurality of sub-carriers; and wherein said soft decision unit responds to said channel estimator to recover the soft decision using the plurality of channel estimates.
 22. The transceiver of claim 20, wherein the soft decision defines soft decision information for each of the plurality of sub-carriers; wherein the hard decision defines hard decision information for each of the plurality of sub-carriers.
 23. The transceiver of claim 22 wherein said signal-to-noise ratio estimation unit comprises: a sub-carrier selection unit to select a subset of the plurality of sub-carriers for the at least one symbol.
 24. The transceiver of claim 23, wherein the subset of the plurality of sub-carriers are regularly spaced within a sub-carrier constellation defined by the plurality of sub-carriers.
 25. The transceiver of claim 24, wherein said FFT unit successively recovers first and second received symbols from the baseband signal, each of the first and second received symbols modulating data across the plurality of sub-carriers; and wherein said sub-carrier selection unit selects first and second subsets of the plurality of sub-carriers, the first and second subsets being at least partially distinct.
 26. The transceiver of claim 23 wherein said signal-to-noise ratio estimation unit comprises: an estimate calculator to generate the geometric signal-to-noise ratio estimate using soft and hard decision information for the subset of the plurality of sub-carriers selected by said sub-carrier selection unit as the soft and hard decision respectively.
 27. The transceiver of claim 26, wherein said estimate calculator and said soft decision unit share common calculation circuitry.
 28. The transceiver of claim 20, wherein the received symbol comprises an OFDM symbol compliant with at least one of IEEE 802.11a and IEEE 802.11g standards.
 29. The transceiver of claim 20, wherein: said soft decision unit comprises a demodulation/FEQ unit; and wherein said hard decision unit comprises one of a slicer and a combination of a Viterbi decoder and a symbol re-coder.
 30. The transceiver of claim 20, further comprising a control unit responsive to said signal-to-noise estimation unit to selectively alter a characteristic of the communications link based on the geometric signal-to-noise ratio estimate generated by said signal-to-noise estimation unit.
 31. A network interface apparatus, comprising: a transceiver, comprising: a receiver capable of receiving a baseband signal borne across a wireless communications link; and a baseband processor responsive to said receiver and comprising: an FFT unit to recover a received symbol from the baseband signal, the received symbol modulating data across a plurality of sub-carriers; a soft decision unit responsive to said FFT unit to recover a soft decision of a transmitted symbol corresponding to the received symbol; a hard decision unit responsive to said soft decision unit to recover a hard decision of the transmitted symbol based on the soft decision; a signal-to-noise ratio estimation unit to generate an estimate of a geometric signal-to-noise ratio for the received symbol based on a function of the soft and hard decisions, wherein a signal quality of the wireless communications link is based on the geometric signal-to-noise ratio; and a decoder responsive to said soft decision unit to recover data corresponding to the soft decision; and a network interface responsive to said decoder to receive the data, said network interface including a control unit responsive to said signal-to-noise estimation unit to selectively alter a characteristic of the wireless communications link based on the geometric signal-to-noise ratio estimate generated by said signal-to-noise estimation unit.
 32. An information processing apparatus, comprising: an information processor; a transceiver responsive to said information processor and comprising: a receiver capable of receiving a baseband signal borne across a wireless communications link; and a baseband processor responsive to said receiver and comprising: an FFT unit to recover a received symbol from the baseband signal, the received symbol modulating data across a plurality of sub-carriers; a soft decision unit responsive to said FFT unit to recover a soft decision of a transmitted symbol corresponding to the received symbol; a hard decision unit responsive to said soft decision unit to recover a hard decision of the transmitted symbol based on the soft decision; a signal-to-noise ratio estimation unit to generate an estimate of a geometric signal-to-noise ratio for the received symbol based on a function of the soft and hard decisions, wherein a signal quality of the wireless communications link is based on the geometric signal-to-noise ratio; and a decoder responsive to said soft decision unit to recover data corresponding to the soft decision; and a network interface responsive to said decoder to receive the data and selectively transmit the data to said information processor, said network interface including a control unit responsive to said signal-to-noise estimation unit to selectively alter a characteristic of the wireless communications link based on the geometric signal-to-noise ratio estimate generated by said signal-to-noise estimation unit.
 33. A method for measuring signal quality in a communications link, the communications link capable of supporting transfer of OFDM symbols modulating data across a plurality of sub-carriers, the method comprising: receiving an OFDM symbol from the communications link; and generating an estimate of a geometric signal-to-noise ratio for the OFDM symbol based on a function of a soft decision and a hard decision for the OFDM symbol, wherein a signal quality of the communications link is based on the geometric signal-to-noise ratio.
 34. The method of claim 33 wherein the soft decision defines soft decision information for each of a plurality of sub-carriers, wherein the hard decision defines hard decision information for each of the plurality of sub-carriers.
 35. The method of claim 34 wherein said generating step comprises: selecting a subset of the plurality of sub-carriers for the OFDM symbol.
 36. The method of claim 35 wherein the subset of the plurality of sub-carriers are regularly spaced within a sub-carrier constellation defined by the plurality of sub-carriers.
 37. The method of claim 36 wherein said selecting step comprises selecting first and second subsets of the plurality of sub-carriers for first and second symbols respectively, the first and second symbols successively transferred across the communications link, the first and second subsets being at least partially distinct.
 38. The method of claim 35 wherein said generating step comprises: generating the geometric signal-to-noise ratio estimate using soft and hard decision information for the subset of the plurality of sub-carriers selected in said selecting step as the soft and hard decision respectively.
 39. The method of claim 33 wherein the communications link comprises a wireless communications link, and wherein the OFDM symbol is formatted in compliance with at least one of IEEE 802.11a and IEEE 802.11g standards.
 40. The method of claim 33 further comprising: slicing the soft decision to provide the hard decision; and demodulating and equalizing the OFDM symbol to provide the soft decision.
 41. The method of claim 33 further comprising: decoding and symbol re-coding the soft decision to provide the hard decision; and demodulating and equalizing the OFDM symbol to provide the soft decision.
 42. The method of claim 33 further comprising selectively altering a characteristic of the communications link based on the geometric signal-to-noise ratio estimate generated in said generating step. 